For many applications it is important to know the operating level, i.e. the flow rate V and the total pressure difference .DELTA.p.sub.t of a fan in its installed state. In refineries, in the chemical industry or in process technology the knowledge of the flow rate V is a requisite for establishing the material balance. In nuclear power plants fans and the like are integrated in the safety chain for keeping an underpressure. In the construction of installations and equipment, for instance in chip drying or the drying of plaster board, the knowledge of the operating levels of the fans improves the control of the installation or the equipment. When fans are used in building technology, the operating levels of the fans are needed for energy management. The conditions in the installed state do not allow for measuring methods which can be compared to measurements taken on the test stand.
A generic method and a generic device for determining the flow rate of a radial fan are known from EP-B 0 419 798. The radial fan has at least one inlet nozzle defining a cross flow opening and arranged upstream of the radial fan wheel. At least partially at one measuring point in the area of the inner periphery of the inlet nozzle and namely in the area preceding its cross flow opening a measuring device is arranged which is designed as a static pressure measuring device and connected via an opening in the nozzle wall close to the measuring point.
The static pressure prevailing in front of the cross flow opening of the inlet nozzle is measured with the static pressure measuring device at the measuring point and compared to the static pressure prevailing in the fan surroundings. The pressure difference .DELTA.p is proportional to the square of the flow rate V. From the measured difference of the pressures .DELTA.p the flow rate V is calculated, and with the aid of the flow rate V the operating efficiency of an already installed fan can be established. The proportionality factor depends on the mounting situation, so that each time it is necessary to include a calibration curve. As a rule in installations there are rarely sufficient working sections for calibrations, since the tendency is to build these installations as compact as possible. A drawback of this method is that fan influx disturbances are not noted. This can lead to wrong interpretations of the measurement results.